Conventionally, when an output current of a power supply device including a switching power supply circuit is equal to or larger than a rated current, in order not to damage the power supply device and a load coupled to the power supply device, control to reduce an output voltage of the power supply device is performed. The control is called drop control.
Examples of methods for detecting an output current of a switching power supply circuit include, for example, the following methods. A first method is a method in which a current detecting resistor is coupled to a secondary winding side of a transformer included in the switching power supply circuit and an output current is detected from a voltage applied to the resistor.
A second method is, for example, a method in which a current transformer circuit is coupled to a primary winding side of a transformer included in the switching power supply circuit and an output current is detected, based on an output voltage of the current transformer circuit.
When an output current of the switching power supply circuit is detected using a current detecting resistor, a loss is large, and therefore, a current transformer circuit with a small loss may be used.
However, when a current transformer circuit is used, due to the influence of a ripple of a switching current, there is a difference between an output current of the switching power supply circuit, which is detected in the current transformer circuit, and an actual output current. Therefore, there is a problem in which an overcurrent is not properly controlled.
The following are reference documents.    [Document 1] Japanese Laid-open Patent Publication No. 2014-119354,    [Document 2] Japanese Laid-open Patent Publication No. 2012-90406, and    [Document 3] Japanese Laid-open Patent Publication No. 2008-113476.